1. Field of the Invention
This invention relates to compounds having a positive dielectric anisotropy which have liquid-crystalline properties, and more particularly to such compounds which have a nematic mesophase.
2. Description of the Prior Art
Liquid crystal displays using twisted nematic cells, also known as TN-displays require, as is wellknown, nematic liquid-crystalline materials having a positive anisotropy of the dielectric constant, also indicated as a positive DCA. The DCA, or .DELTA..epsilon., defined as .DELTA..epsilon.=.epsilon..sub." -.epsilon..sub..perp. wherein .epsilon..sub." is the dielectric constant (DC) parallel to the long axis of the molecule and .epsilon..sub..perp. is the DC perpendicular to the molecular axis.
For multiplexed operation of these cells it is also desirable that the LC-material have a characteristic curve exhibiting the steepest possible contrast, i.e., the greatest possible change of the light absorption properties with change of the applied electric field. This can be attained by optimizing two criteria used in choosing the available anisotropic materials, namely (a) the smallest possible value of the ratio of the elastic constants of bending and of expansion, also indicated as k.sub.33 /k.sub.11, and (b) by the smallest possible value of the ratio of the DCA (.DELTA..epsilon.) to the DC perpendicular to the nematic axis (e.sub..perp.), i.e., of .DELTA..epsilon./.epsilon..sub..perp.. The particulars of these requirements can be found in the literature: Gharadjedagji, F. et al., Rev. Phys. Appl., 11 (1976), 467; Metz, A. R., SID Digest, Techn. Papers IX (1978), 70 and Alt, P. M. et al., IEEE Trans. El. Dev. Ed. (1974), 146.
At present, two types of anisotropic compounds are available from which to select suitable anisotropic compounds by the above criteria. Each type may contain two or three rings, and they have the following structure: ##STR2## wherein a.sup.1, a.sup.2 and a.sup.3 are aromatic, heteroaromatic or cycloaliphatic rings, such as benzene, pyrimidine, and trans-cyclohexane rings; X.sup.a and Y.sup.a are so-called bridge groups such as --COO--, --CH.dbd.N--, --CH.sub.2 O--, --N.dbd.N--, --CH.sub.2 CH.sub.2 -- or a single bond and R.sup.a , R.sup.b are so-called terminal groups, such as alkyl, alkoxy, alkanoyloxy, or alkylamino groups, which contain an alkyl portion having in general 1 to 12 C atoms in a straight or branched chain.
Type I substances, because of their relatively large polarity R.sup.a /CN in the direction of the long axis of the molecule (the nematic axis or director) offer a high positive DCA-value and a correspondingly low threshold voltage. However, they have high .DELTA..epsilon./.epsilon..sub..perp. -values, which is disadvantageous for multiplex operation and also have a comparatively high k.sub.33 /k.sub.11 -value which is also disadvantageous for this purpose. The contrast characteristic curve of these substances is relatively weak.
Type II substances are slightly polar or nonpolar and have .DELTA..epsilon./.epsilon..sub..perp. -values necessary for a steep characteristic curve, and k.sub.33 /k.sub.11 -values in comparison with substances of Type I (at comparable chain length). Furthermore, the viscosity of Type II substances is less than that of Type I substances.
For all these reasons, anisotropic substances of Type II are indispensible components of LC-mixtures for multiplex operation of TN-displays.
However, a significant disadvantage of the known anisotropic substances of Type II is their pronounced tendency to form smectic phases, especially when these materials have relatively long terminal groups, e.g., when they have alkyl groups of a certain length. This can be seen by referring to the values in the literature, e.g. in "Liquid Crystals in Tables," VEB Deutscher Verlag, Leipzig, 1974.
The appearance of smectic phases in anisotropic compounds narrows the range of the nematic phase which alone is suitable for TN-displays or completely eliminates it.
Therefore, a need has continued to exist for anisotropic compounds of Type II having a reduced tendency to form smectic phases.